doc/go1.11: add note about package versioning

It will not be present in go1.11beta1 but will be present
in subsequent releases.

Change-Id: I298fb682945345bb4a34ec83802fd644f75bdd98
Reviewed-on: https://go-review.googlesource.com/120756Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.11.html

@@ -53,6 +53,15 @@ Do not send CLs removing the interior tags from such phrases.
   TODO: PPC64LE race detector support
 </p>
 
+<h3 id="package-versioning">Package Versioning (vgo)</h3>
+<p>
+  <strong>
+    NOTE: This is not present in go1.11beta1 but will be available in future
+    betas and subsequent releases.
+  </strong>
+  Go 1.11 adds experimental, integrated support for package versioning.
+</p>
+
 <h3 id="wasm">WebAssembly</h3>
 <p>
   Go 1.11 adds an experimental port to WebAssembly (<code>js/wasm</code>).